Copper alloys are widely used in electrical, electronic and architectural applications. The alloys have excellent electrical characteristics but the mechanical properties are less than ideal. The alloys are soft and subject to frictional and erosive wear. The alloys readily oxidize and because copper oxide is a poor conductors of electricity, the oxide causes an increase in contact resistance.
Oxidation can be minimized by coating the alloy with a second metal such as tin or gold. However, the coatings do not improve the hardness or wear resistance of the substrate.
Various alloying elements may be added to molten copper to increase wear resistance and hardness as well as to improve oxidation resistance. The additions usually cause a decrease in electrical conductivity.
As disclosed in U.S. Pat. No. 5,096,508 07/563,766, which is incorporated by reference in its entirety herein, the surface of a copper alloy may be modified by nitriding. The process changes the characteristics of the surface to a depth of from a few angstroms to about 1 micron.
U.S. Pat. No. 4,693,760 issued to Sioshansi discloses modifying the surface of a titanium alloy with carbon or nitrogen by ion-implantation. An article by Oliver et al entitled The Wear Behavior of Nitrogen-implanted Metals details how the wear characteristics of titanium alloys, hard chromium plate and ferrous metals are improved by nitriding the surface by ion-implantation.
Elemental copper has low reactivity with nitrogen and carbon and nitriding or carbiding by conventional means, such as used to nitride steel, does not produce a significantly improved surface.
A boride modified copper surface is disclosed in U.S. Pat. No. 4,436,560 to Fujita et al. A copper substrate is alloyed with a metal which reacts with boron. Boron is brought into contact with the alloy by immersion in a molten boron salt, fluid bed deposition or vapor deposition. The resultant surface has a dispersion of fine boride particles in a copper matrix. The dispersoids occupy from 1 to 50% of the surface volume.
The technique of U.S. Pat. No. 4,436,560 is only suitable when the precipitate former (boron) is soluble in the host matrix (copper). Both carbon and nitrogen are not significantly soluble in copper and the process is not suitable for forming nitride or carbide precipitates at the surface of a copper alloy.
A method of modifying the surface of alloy steels is disclosed in U.S. Pat. No. 4,692,191 to Maeda et al. The patent discloses immersing the substrate in an oxidizing acid or salt and irradiating selected surface areas with a laser pulse The irradiated surface formed a passive oxide surface layer. The oxide may be a different color than the base metal and by irradiating selected portions of the substrate, a polychromatic surface is obtained.